Experimental and theoretical investigation of the H(H)over-bar1 Sigma(+)(g) state and the B ''(B)over-bar (1)Sigma(+)(u) state in HD

We present an extensive experimental and theoretical study of the energy le vels of the (H) over bar outer-well part of the H (H) over bar (1)Sigma(g)( +) state, which is the fourth adiabatic state of (1)Sigma(g)(+) symmetry in the hydrogen molecule. Experimentally, rovibrational H levels were excited in a two-step laser excitation, using wavelength tunable extreme ultraviol et radiation near 91 nm to prepare selected levels in the B (1)Sigma(u)(+) state (upsilon = 18 and 19 for H-2 and upsilon = 25-27 for D-2, upsilon = 2 1-23 for HD). A second tunable laser in the range 550-735 nm was used to ex cite the (H) over bar levels in the isotopomers H-2, D-2, and HD. A third l aser at 355 nm probed the excitation of the (H) over bar levels by dissocia tive ionization, producing ions for signal detection. For H-2 82 quantum le vels were calibrated (upsilon = 2-15, J = 0-5) and 107 levels for D-2 (upsi lon = 6-22, J = 0-5). These level energies are compared with ab mitio calcu lations including adiabatic and relativistic effects. Agreement between obs ervation and calculation is of the order of 1 cm(-1) for H-2 and 0.5 cm(-1) for D-2 throughout the rovibrational manifold. In the HD isotopomer the (H ) over bar (1)Sigma(g)(+) state nonadiabatically interacts with the (B) ove r bar (1)Sigma(u)(+) state. This mixing represents an example of strong g-u symmetry breaking. Apart from (H) over bar levels (v = 4-19, J= 0-3) also B levels (upsilon = 9-20, J = 0-3) were observed. Theoretical results for I -ID account for the nonadiabatic interaction with the B state, based on a n ew ab initio calculation of the B "(B) over bar (1)Sigma(u)(+) potential an d the coupling with the H (H) over bar (1)Sigma(g)(+) state. Except for som e levels near the potential barrier also for HD the experimentally calibrat ed levels agree with the ab initio calculated energies within 1.5 cm(-1). [ S1050-2947(99)06007-2].